The present invention is based on a fuel injector according to the species defined in claim 1.
A fuel injector according to the definition of the species in claim 1 is known from German Patent No. 195 00 706 A1. The fuel injector proceeding from this printed publication provides a piezoelectric actuator for actuating a valve needle connected to a valve-closure member. The valve-closure member interacts with a valve-seat surface to form a sealing seat. In this context, both the embodiment of a fuel injector opening to the outside and the embodiment of a fuel injector opening to the inside are possible. The piezoelectric actuator made of a plurality of stacked piezoelectric layers does generate relatively large lifting forces, but relatively small lift heights. Therefore, the named publication proposes providing a hydraulic transmission device between the valve needle and the piezoelectric actuator, in order to increase the lift transmitted to the valve needle.
A disadvantage of this known design of a fuel injector having a piezoelectric actuator is, that the relatively large volume and the relatively large cross-sectional area of the actuator does not allow one to realize an especially compact design. In addition, it is disadvantageous that a special hydraulic medium is used for the transmission device, which can volatize over time due to leakage. This can impair the functioning of the transmission device, and affect the service life of the fuel injector.
Another design of a fuel injector having a piezoelectric actuator is known from German Patent No. 43 06 073 C1. In this fuel injector, the movement of the piezoelectric actuator is transformed into the movement of the valve needle, using a hydraulic transmission device as well. This fuel injector also has the disadvantage of a relatively voluminous design that is not very compact, and the disadvantage of leaking hydraulic medium.
In contrast, the fuel injector according to the present invention, having the features of claim 1, has the advantage that the tubular form of the actuator, which at least sectionally encircles the valve needle or an actuating element for actuating the valve needle, achieves an especially compact and inexpensive design. The inner volume of the tubular, piezoelectric or magnetostrictive actuator can be used to receive component parts which, in the case of the known piezoelectric actuator, are situated in the axial extension of the piezoelectric actuator. Furthermore, tubular actuators can be manufactured relatively inexpensively. The fuel injector according to the present invention is especially suited for direct injection of gasoline into the combustion chamber of an internal combustion engine, since the fuel pressures occurring there lie in the range of approximately 100 to 200 bar and, relatively speaking, are markedly lower than those in diesel fuel injectors. Therefore, the actuating forces to be applied by the piezoelectric or magnetostrictive actuators in these direct gasoline-injection valves are also less than those in diesel fuel injectors, so that the actuating force for actuating these fuel injectors, which is conditional upon the tubular design, and reduced in comparison with an actuator made of solid material, is still perfectly sufficient.
Advantageous further refinements and improvements of the fuel injector indicated in the main claim are rendered possible by the measures specified in the dependent claims.
It is advantageous, when a hydraulic transmission device is provided between the valve needle and the actuator, and the actuator encircles a transmission piston acting on the transmission device. In order to connect it to the actuator, the transmission piston preferably has a flange, which is on the side opposite to the sealing seat, supports the actuator, and is preferably bonded to the actuator. The transmission piston can be integrated in the tubular actuator, which results in a compact design.
The actuator is preferably prestressed by a first prestressing spring, and the valve-closure member and the valve needle are reset by a restoring spring that is independent thereof
If the fuel injector is a fuel injector that opens to the inside, it is advantageous when the transmission device reroutes the force between the transmission piston and the valve needle. Therefore, depending on the configuration of the transmission device, both fuel injectors opening to the outside and fuel injectors opening to the inside are feasible with the same general type of actuator construction. To divert the force for a fuel injector opening to the inside, the transmission device advantageously includes a housing member having an inner opening, in which the transmission piston can be moved. In this context, the housing member is enclosed a by the valve needle or a coupling piece, an inner chamber being formed between the transmission piston and the housing member, and an outer chamber being formed between the valve needle or coupling piece and the housing member, the inner chamber and the outer chamber being interconnected so as to communicate with each other. This forms an especially compact transmission device, which can be manufactured particularly inexpensively.
The fuel, which is conveyed in the fuel injector and is to be ejected by the fuel injector, is preferably used as a hydraulic medium for the transmission device. Therefore, the fuel injector must not be filled with a special hydraulic medium, e.g. a hydraulic oil, which can escape over time, due to leaking. Rather, fuel used as a hydraulic medium is automatically and continually refilled through guide openings, in a quasistatic manner.